The transportation of oils with high cloud points by pipelining can result in the deposition of wax at the pipewall if the oil properties are such that wax precipitates out of solution with the oil at temperatures above the surroundings of the pipeline. In this type of situation, wax will deposit at the pipewall where the oil cools to below its cloud point. One method for prevention of wax deposits in this manner is to pre-cool the oil to, at, or below the coldest wall temperature prior to the oil entering the pipeline. The wax is left in the oil stream. The wax then flows in the pipeline as a slurry with the oil. Thus, as the system is designed, the oil, wax, and pipeline are at essentially the same temperature, the wax will not deposit on the pipe wall. In addition to preventing wax deposits, another benefit of operating a "cold" pipeline, particularly in severely cold environments, is the protection of the frozen soil or permafrost from thawing by a heated, possibly insulated, pipeline. The problem of thaw subsidence due to melting the permafrost is eliminated by operating a pipeline at the same temperature as the frozen soil. The usual method for precooling the oil is with heat exchangers or chillers. However, the problem of wax deposition is then transferred to the heat exchangers or chillers rather than the pipeline.
U.S. Pat. No. 3,454,464 discloses the choke cooling of a petroleum stream in a production well to restrict paraffin deposition. The following U.S. patents are also considered of relevance to the present invention: U.S. Pat. Nos. 3,027,319; 2,303,823; Re. 30,281; Re. 25,759. Also considered of relevance are British Pat. Nos. 768,655 and 768,654.
Commonly assigned application Ser. No. 868,920, filed May 29, 1986 is relevant to the present application.